1. Field of the Invention
This invention generally relates to an arrangement for, and a method of, detecting the presence or absence of persons or things in an area, such as an entranceway, both outdoor and indoor, and, more particularly, to fail-safe obstruction detectors especially useful in systems for controlling garage doors, elevator doors and the like, and to security systems for controlling access to a secure area.
2. Description of the Related Art
An obstruction detector for an automatic garage or elevator door control system prevents personal injury and/or property damage to a person or thing caught in the closing door, as well as preventing damage to the door itself and the various drive components that close the door.
As shown in FIGS. 1-2, it has been proposed to use a light transmitter 1 powered by a controller 7 to transmit a light beam, along a direct path, across a door opening or entranceway for detection by a light receiver 2. Unobstructed receipt of the light beam indicates that the entranceway is free of the object 3. However, failure to receive the light beam indicates that the entranceway has an obstruction. This failure is electronically processed by the drive controller 7 to either stop or reverse a closing door. Analogously, it is known in security systems to transmit a light beam across a window or security zone. Interruption of the beam triggers an alarm, thereby summoning law enforcement personnel.
As shown in FIGS. 3-4, it has also been proposed to use the light transmitter 1 powered by the controller 7 to transmit the light beam across a door opening for reflection by a retroreflector 4 back toward the receiver 2 along a folded path. Receipt of the light beam at a certain power level indicates the absence of an obstruction, whereas receipt of the light beam at a different power level indicates the presence of the obstruction. One limitation of such folded path systems is that when an obstruction 5 is relatively near the receiver 2, the power level reflected by the obstruction 5 can be the same as the power level reflected by the retroreflector 4. The system can be fooled into thinking that no obstruction is present. To alleviate this problem, the art has employed a polarizer 6 for differentiating between reflections by the obstruction and the retroreflector. However, even a polarizer can be tricked by a diffused reflective object at close range.
Other prior patents of which I am aware include U.S. Pat. No. 5,473,461 and U.S. Pat. No. 5,508,511, in which I am a named inventor.